In approximately one in five cases, the infection and source of sepsis cannot be detected.

What causes the symptoms of sepsis?

Usually, your immune system will keep the infection limited to one place (known as a localised infection). Your body will produce white blood cells, which travel to the site of the infection to destroy the germs causing infection. A series of biological processes occur, such as tissue swelling, which helps fight the infection and prevents it spreading. This process is known as inflammation.

If your immune system is weakened or an infection is particularly severe, it can spread through the blood into other parts of the body. This causes the immune system to go into overdrive, and the process of inflammation affects the entire body.

This can cause more problems than the initial infection, as widespread inflammation damages tissue and interferes with the flow of blood, leading to a dangerous drop in blood pressure, which stops oxygen reaching your organs and tissue.

People at risk

Everybody is potentially at risk of developing sepsis from minor infections, such as flu. However, some people are more vulnerable, including people who:

have a medical condition, such as HIV or leukaemia, that weakens their immune system

are receiving medical treatment, such as chemotherapy, that weakens their immune system

are very young or very old

have just had surgery, or have wounds or injuries as a result of an accident

are on mechanical ventilation

with drips or catheters attached to their skin

are genetically prone to infection

Sepsis is a particular risk for people already in hospital due to another serious illness. Despite the best efforts of doctors and nurses, secondary infections acquired in hospital are always a potential risk.

Hospital-acquired bacterial infections, such as MRSA, tend to be more serious as the bacteria causing the infection have often developed a resistance to antibiotics.